The present application relates to a memory device including a memory layer which uses the magnetization state of a ferromagnetic layer to store information and a fixed magnetic layer whose magnetization direction is fixed. The memory device is configured so that a current is applied thereto to change the magnetization direction of the memory layer. The application also relates to a memory including the memory device. The application is suitable for the application to a nonvolatile memory.
A high-speed, high-density DRAM has been widely used as a random access memory in information equipment, such as a computer.
However, a DRAM is a volatile memory that loses stored information when the power is turned off, and thus a nonvolatile memory that retains the stored information has been desired.
As possible nonvolatile memories, attention is focused on magnetoresistive random access memories (MRAMs) that use the magnetization of a magnetic material to store information, which have been under development (see, e.g., Nikkei Electronics, No. 2001-2-12 (pp. 164 to 171)).
In an MRAM, current is passed through two kinds of addressing wires arranged at substantially right angles to each other (a word line and a bit line). The current magnetic field generated from each addressing wire reverses the magnetization of a magnetic layer of the magnetic memory device disposed at the intersection of the addressing wires, whereby information is stored in the magnetic layer.
FIG. 6 shows a schematic diagram (perspective view) of an ordinary MRAM.
In a region isolated by an element isolation layer 102 on a semiconductor base 110 such as a silicon substrate, a drain region 108, a source region 107, and a gate electrode 101 are formed to provide a selection transistor for selecting a memory cell.
Above the gate electrode 101, a word line 105 is provided to extend in the anteroposterior direction in the figure.
The drain region 108 is common between right and left selection transistors in the figure, and a wire 109 is connected to the drain region 108.
Further, a magnetic memory device 103 is disposed between the word line 105 and a bit line 106 provided thereabove to extend in the horizontal direction in the figure. The magnetic memory device 103 has a memory layer whose magnetization direction is to be reversed. A magnetic tunnel junction element (MTJ element) may be used to form the magnetic memory device 103, for example.
Further, the magnetic memory device 103 is electrically connected to the source region 107 via a horizontal bypass line 111 and a vertical contact layer 104.
A current is passed through each of the word line 105 and the bit line 106. As a result, a current magnetic field is applied to the magnetic memory device 103, whereby the magnetization direction of the memory layer of the magnetic memory device 103 is reversed to allow information to be stored therein.
In order for stored information to be stably retained in a magnetic memory such as an MRAM, a magnetic layer to store the information (memory layer) is required to have certain coercivity.
Meanwhile, for rewriting the stored information, it is necessary to pass a certain amount of current through the addressing wires.
However, as MRAM-forming elements decrease in size, the current value needed to reverse magnetization has been increasing, whereas because of thinner addressing wires, it is getting more difficult to pass a sufficient current therethrough.
Accordingly, memories that use magnetization reversal by spin injection have been attracting attention as memories that enable magnetization reversal at lower current (see, e.g., JP-A-2003-17782; U.S. Pat. No. 6,256,223 specification; PHYs. Rev. B, 54., 9353, (1996); and J. Magn. Mat., 159., L1, (1996)).
Magnetization reversal by spin injection represents a process in which electrons spin-polarized through a magnetic material are injected into a different magnetic material to thereby cause magnetization reversal in the different magnetic material.
For example, by passing a current through a giant magnetoresistance element (GMR element) or a magnetic tunnel junction element (MTJ element) in the direction perpendicular to the film plane, the magnetization direction of at least one of the magnetic layers in the element can be reversed.
Magnetization reversal by spin injection is advantageous in that even in a minute element, magnetization reversal can be achieved without the need for increased current.
FIGS. 7 and 8 show schematic diagrams of such a memory that uses magnetization reversal by spin injection. FIG. 7 is a perspective view, and FIG. 8 is an sectional view.
In a region isolated by an element isolation layer 52 on a semiconductor base 60 such as a silicon substrate, a drain region 58, a source region 57, and a gate electrode 51 are formed to provide a selection transistor for selecting a memory cell. The gate electrode 51 also serves as a word line that extends in the anteroposterior direction in FIG. 7.
The drain region 58 is common between right and left selection transistors in FIG. 7, and a wire 59 is connected to the drain region 58.
Further, a memory device 53 is disposed between the source region 57 and a bit line 56 provided thereabove to extend in the horizontal direction in FIG. 7. The memory device 53 has a memory layer whose magnetization direction is to be reversed by spin injection.
A magnetic tunnel junction element (MTJ element) may be used to form the memory device 53, for example. The reference numerals 61 and 62 indicate magnetic layers. Of the two magnetic layers 61 and 62, one magnetic layer serves as a fixed magnetic layer having a fixed magnetization direction, while the other magnetic layer serves as a free magnetic layer whose magnetization direction is variable, i.e., a memory layer.
The memory device 53 is connected to the bit line 56 and the source region 57 via upper and lower contact layers 54, respectively. Accordingly, a current can be passed through the memory device 53 to reverse the magnetization direction of the memory layer by spin injection.
In the case of such a memory that uses magnetization reversal by spin injection, as compared with the ordinary MRAM shown in FIG. 6, the device structure can be simplified, whereby higher density can be achieved.
Further, as compared with magnetization reversal using an external magnetic field as in ordinary MRAMs, magnetization reversal by spin injection also provides an advantage in that the write current is not increased even when the device decreases in size.
Incidentally, in an MRAM, write lines (word lines and bit lines) are provided separately from memory devices. A current is passed through the write lines, and the thus-generated current magnetic field is used to write (store) information. Therefore, a sufficient amount of current required for writing can be passed through the write lines.
In a memory that uses magnetization reversal by spin injection, a current is passed through a memory device to achieve spin injection and thereby reverse the magnetization direction of a memory layer.
Because a current is directly passed through memory devices in this way to write (store) information, for the selection of a memory cell for writing, each memory device is connected to a selection transistor to form a memory cell. In such a case, the current passed through the memory device is limited by the magnitude of current that can be passed through the selection transistor (the saturation current of the selection transistor).
Therefore, it is necessary to write at a current not more than the saturation current of the selection transistor, and it thus is necessary to improve the spin injection efficiency to reduce the current passed through the memory device.
Further, in order to increase a readout signal, it is necessary to ensure a high magnetoresistance change ratio. For this purpose, a memory device configured to have tunnel insulating layers (tunnel barrier layers) as intermediate layers on the opposite sides of a memory layer is effective.
However, in such a case where tunnel insulating layers are used as intermediate layers, in order to prevent the tunnel insulating layers from dielectric breakdown, the amount of current passed through the memory device is limited. In these respects, it will be necessary to (1) ensure a high magnetoresistance change ratio, (2) suppress the spin injection current, and (3) increase the breakdown voltage of the memory device.
A possible measure to suppress the spin injection current to meet one of the above conditions is to apply a dual-pin structure to a memory device, in which two fixed magnetic layers are positioned above and below a memory layer with intermediate layers therebetween, respectively.
In connection with such a dual-pin structure, for the purpose of suppressing the spin injection current, a structure has been proposed in which ferromagnetic layers of the two fixed magnetic layers closest to the respective intermediate layers have magnetization directions antiparallel to each other (see, e.g., JP-A-2004-193595 and JP-A-2006-269530).